docs: def supported_alleles(self):
          """
          Alleles for which predictions can be made.
          
          Returns
          -------
          list of string
          """
          if 'supported_alleles' not in self._cache:
              result = set(self.allele_to_allele_specific_models)
              if self.allele_to_fixed_length_sequence:
                  result = result.union(self.allele_to_fixed_length_sequence)
              self._cache["supported_alleles"] = sorted(result)
          return self._cache["supported_alleles"]